From EP 0 560 616 A1 an exhaust system having a hollow jacket is known. An intermediate space delimited by the walls of the hollow jacket can be subjected to a vacuum. For this purpose, the intermediate space of the hollow jacket is connected to the air intake of the combustion engine via a vacuum connecting line. Accordingly, when the combustion engine is in operation, its intake of air the fluid present in the intermediate space of the jacket is also sucked out, as a result of which a vacuum is created in this intermediate space. This vacuum generation can be controlled through a vacuum control valve which is arranged in the vacuum connecting line. With this vacuum control valve the intermediate space of the hollow jacket can also be ventilated.
From WO 00/43103 and U.S. Pat. No. 6,162,403 catalytic converters each with an evacuated double-walled jacket are known. In addition, in an evacuated region of the jacket, a material which through a phase transformation at a predetermined temperature can absorb or emit heat is likewise arranged. Thus, upon further heating and also short-term cooling through the phase transformation of the material the catalytic converter is held at a constant temperature for as long as a phase transformation of the material is still possible.
The catalytic converter of U.S. Pat. No. 6,203,764 B1 likewise comprises a double-walled evacuated jacket. In the jacket, hydrogen-absorbing material is arranged. At low temperature the greatest part of the hydrogen is bound by the hydrogen-absorbing material and a vacuum is present in the double-walled jacket. When the catalytic converter heats up during the operation the hydrogen-absorbing material starts to release the hydrogen from a predefined temperature, as a result of which the vacuum in the double-walled jacket drops. Thus, the double-walled jacket at a lower temperature has a higher insulating effect than at higher temperature because of the falling vacuum and the heat convection through the hydrogen molecules that have been liberated.
The exhaust gas re-treatment, particularly the catalytic exhaust gas re-treatment develops its optimum effectiveness only within a predefined temperature interval. Outside this predefined temperature range the effectiveness of the exhaust gas re-treatment falls and/or expires completely. Accordingly, it is necessary particularly with the catalytic components of the exhaust system or also with the entire exhaust system to reduce the heat losses occurring on the exhaust system through insulation. By means of this, the exhaust system and its components can be heated up more quickly particularly during cold starting and among other things the catalytic exhaust gas re-treatment function optimally.
This is particularly desirable in the load ranges, in which the exhaust gas is relatively cold, such as for example during a cold start. In these load ranges a best possible insulation is desired in order to reduce or avoid further cooling down of the exhaust gas. By doing so, the exhaust emission values can be significantly improved and the demanded emission regulations can be adhered to.
However, load ranges wherein the exhaust gas is too hot and the predefined temperature interval of the exhaust gas re-treatment is thus exceeded, can also occur. However, in this case a reduced functionality of the components of the exhaust system in terms of the exhaust gas re-treatment can also occur and consequently at these load points a cooling down of the exhaust gas is not only desirable but even necessary in certain cases. Accordingly, a reduced insulation of the exhaust system and its components is advantageous in these load ranges.